In the winter of 1925, the dog sled relay was a way of life in remote northern Alaska…and during that season, it also became a matter of life and death. Aircraft had yet to become reliable enough for the punishing conditions of the region, so mushing teams were responsible for delivering mail during the coldest months of the year. So when an outbreak of diphtheria threatened to decimate the small city of Nome, where Alaskan Natives had no natural immunity, it was up to a team of dogs and their sled drivers to deliver antitoxin to the one doctor in town, whose batch had expired. Without it, the estimated mortality rate was around 100 percent.

The “Great Race of Mercy,” as it came to be called, actually consisted of two relays. The first delivered a supply of 300,000 units of antitoxin to Nome. The supply had been found in Anchorage. It wasn’t enough to wipe out the outbreak, but it could hold it at bay while a supply of 1.1 million units was rounded up along the West Coast, shipped to Seattle, and delivered to the Alaska Territory. The trip from Nenana to Nome (which was as far as the trains could deliver supplies into the Alaskan interior) normally took 25 days by dog sled. The antitoxin would only survive for about six days along the trail. Facing some of the worst conditions on the planet – with hurricane-force winds and temperatures consistently below -50 F – a team of 20 drivers and around 150 dogs covered the 674 miles in 5 ½ days from January 27 to February 2.

Two weeks later, the final shipment of antitoxin arrived in Nome. The 1.1 million units that would wipe out what threatened to become an epidemic arrived in Nome 90 years ago today on February 15, 1925. The second relay had taken a relatively leisurely seven days. The death toll from the diphtheria outbreak was officially listed at 5-7 people, but was likely higher since Alaskan Natives had a custom of burying their dead without alerting the authorities. A number of dogs also died delivering the antitoxin. Balto, the lead dog of the final leg of the relay into Nome, became emblematic of the heroic animals. He became a celebrity in his own right, and had a statue erected in New York’s Central Park. Today the Iditarod Dog Sled Race honors the serum run and the history of the dog sled in Alaska, which saw its last great hurrah come to an end on this date.